Thermal imaging or thermography is a recording process wherein images are generated by the use of imagewise modulated thermal energy. Thermography is concerned with materials which are not photosensitive, but are sensitive to heat or thermosensitive and wherein imagewise applied heat is sufficient to bring about a visible change in a thermosensitive imaging material, by a chemical or a physical process which changes the optical density.
Most of the direct thermographic recording materials are of the chemical type. On heating to a certain conversion temperature, an irreversible chemical reaction takes place and a coloured image is produced.
In direct thermal printing, said heating of the recording material may be originating from the image signals which are converted to electric pulses and then through a driver circuit selectively transferred to a thermal printhead. The thermal head consists of microscopic heat resistor elements, which convert the electrical energy into heat via the Joule effect. The electric pulses thus. converted into thermal signals manifest themselves as heat transferred to the surface of the thermal paper Wherein, the chemical reaction resulting in colour development takes place. This principle is described in "Handbook of Imaging Materials" (edited by Arthur S. Diamond--Diamond Research Corporation --Ventura, Calif., printed by Marcel Dekker, Inc. 270 Madison Avenue, New York, ed 1991, p. 498-499).
A particular interesting direct thermal imaging element uses an organic silver salt in combination with a reducing agent. Such combination may be imaged by a suitable heat source such as e.g. a thermal head, laser etc. .DELTA. black and white image can be obtained with such a material because under the influence of heat the silver ions are developed to metallic silver.
However, when imaged with a thermal head it appears to be difficult to obtain a neutral black tone image. For this purpose it has been suggested to add toning agents, but these still do not yield satisfactory results. Furthermore, it appears to be difficult to obtain a desired number of grey levels which may be required for some application, in particular if the image is to be used for medical diagnostic purposes.
On the other hand, other applications may require only dense black image on a white background. Such applications are those in which only line-art is to be reproduced e.g. bar code printing, facsimile hard copy generation etc. These applications generally require a neutral black image of high density.